In many electrical applications, it is desirable to connect an electrical component having a plurality of conductive terminals to a substrate including a plurality of laterally spaced conductive paths such that the conductive terminals are aligned with the conductive paths in a predetermined pattern. For proper function, current is passed from the aligned conductive terminal to the substrate path, or visa a versa. It is undesirable for current to pass between laterally spaced terminals or conductive pad, such conductivity causing shorting of the circuit. This desired function is referred to as uniaxial conductivity.
Conventional methods of connecting the conductive terminals to the substrate generally include soldering techniques. Soldering requires dot deposition; that is, application of the solder only on the terminals and not therebetween. Soldering further requires specific temperature resistant components and substrates. Such materials must withstand the 400.degree. F. and greater temperatures generally encountered in the solder application.
The U.S. Pat. No. 4,113,981 to Fujita et al, issued Sept. 12, 1978, discloses an electrically conductive adhesive for connecting arrays of conductors. The adhesive includes a nonconductive base and electrically conductive particles incorporated into the base. The patent requires that the particles are not in contact with one another. The adhesive provides conductivity in one direction between two facing members but does not provide electric conductivity in the lateral direction. The Fujita et al patent requires a mixing ratio of between thirty to sixty percent by volume of conductive particles in the adhesive base. When the proportion of the conductive particles is less than thirty percent by volume, the value of the electrical resistance in the lateral direction begins to be lowered and, when the amount of conductive particles is about sixty percent by volume, a substantial conductivity is manifested in the lateral direction. The patent states that it is preferred that the mixing ratio of the conductive particles be lower than about thirty percent by volume. Additionally, the patent states that the mixing ratio is determined within the above range so that at least one electrically conductive particle is present between opposed facing members to be electrically connected with each other. In practice, such metal particles are chosen to be within the range of approximately 8 to 10 microns. This method therefor requires that the terminal members be brought to within 8 to 10 microns of the substrate paths in order to provide for connection between the terminal member, the electrical particle, and the conductive paths. Such practice requires clean room techniques because in such a microscopic size range, dust particle contamination must be prevented. Therefore, the Fujita et al patent is dependent upon the concept that the space in between the conductive elements must be the same size as the largest of the conductive particles in the adhesive and that the conductive particles in the adhesive must be substantially the same size and shape. Consequently, there is a need for a uniaxially conductive adhesive not requiring the close tolerances or clean room limitations of prior art adhesives.
It has been reported that the conductivity of an adhesive resin having a conductive metal mixed therein is dependent upon the concentration of the adhesive. Polymer Science Technology, Vol. 15, Plenum Press (1981). The instant invention applies this concept to derive a uniaxial conductive adhesive.